A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing
binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are
one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic
variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular
periods.
A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique
known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute
scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then
plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number
of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the
amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.
More recently, the search for extrasolar planets (over 700 discovered so far) has identified yet another interesting
application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an
exoplanet are studied. Further details for the interested party are available
here.
Note: The light curve for exoplanet HAT-P-19b in Andromeda depicted below is one of the
latest transitting exoplanets, having being announced late July/2010, and represents the nineteenth discovery by the
Hungarian-based HATNet Project team. HAT-P-19b is characterized with a low mass of only 0.292 times that of Jupiter
and in spite of a larger radius (1.132 RJup), thus making this find a low-density Saturn-mass exoplanet
with a negligible core mass. Furthermore, radial velocity measurements and the consequent residuals following
model-fitting strongly suggest the presence of another body. HAT-P-19b requires 170 minutes to transit its parent
star at a depth of 21.5 mmag (2.15%).
The parent star, GSC 2283:589, is a K1 dwarf estimated to have a mass of 0.84 solar masses, a radius equivalent to
0.82 solar radii, a temperature of 4,990° K and to lie at a distance of 700 light-years away with a visual magnitude
of 12.901. Further details regarding HAT-P-19 and HAT-P-19b are available in the paper published by the discovery
team led by Hartman et al here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 2283:664 (mag 10.9) and GSC 2283:1197 (mag 12.4) respectively.
Parent Star: HAT-P-19 GSC/SAO Catalog: GSC 2283:589 Constellation: Andromeda RA / Dec: 00h 38m 04s / +34° 42' 42" Magnitude: 12.901 Distance: 700 light-years Exoplanet: HAT-P-19b Period: 4.008778 + 0.000006 d Transit Duration: 170 mins Transit Depth: 21.5 mmag Minimum Mass: 0.292 MJup Radius: 1.132 RJup Pred Transit Details:
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Date: Nov 19-20, 2011 18:30:00 - 23:01:38 UT+2 Location: Athens, Greece Equipment: AP 305/f3.8 Riccardi-Honders AP 1200GTO GEM SBIG ST-10XME SBIG CFW10 SBIG LRGB filters Integrations:
Temperatures:
Software: CCDSoft V5.00.201 AIP4Win V2.2 Processing: Reduction Differential Photometry |